1. Field of the Invention
The present invention relates to methods of using radiation to pattern an electrode of an organic electronic device and to substrates for and processed by these methods. A preferred embodiment relates to a method of using laser radiation to isolate organic pixel electrodes on top of a multilayer device structure containing an array of active electronic devices such that the underlying layers are substantially unharmed and substantially no debris is generated.
2. Description of the Related Art
Backplanes for active matrix displays comprise an array of transistors that are individually addressed by interconnect lines perpendicular to an array of data lines. By driving both the interconnect lines and the data lines, a pixel pad or electrode is charged and a portion of the display medium above this pixel electrode will switch.
One problem that is shared between conventional thin film transistor (TFT) technology and organic semiconductor-based printed TFTs alike is the limited display area, in which the thin film transistor, the gate line, the pixel capacitor and the pixel itself compete with each other for space. This can lead to a reduction in the aperture ratio and therefore the quality of the display. The aperture ratio of the display is defined by the area of the pixel electrode divided by the area of the pixel footprint. Since the pixel electrode is competing with the TFT, interconnects and pixel capacitor for space in the pixel footprint, it is preferable to use a multi-level structure where the pixel electrode is defined on a different layer from the gate interconnect and data lines. When fabricating such an electrode it is advantageous to use an organic conductor because it can be processed from solution, which allows for low cost deposition techniques.
In a conventional display the silicon TFT source-drain electrodes, addressing lines and the pixel electrodes are on one metal level of the device, and the gate electrodes and gate addressing lines are on a second metal level of the device. In conventional display architectures, printed electrodes and printed pixel capacitors tend to be large and result in active matrix displays with low aperture ratios.
In our preferred structure, disclosed in patent number PCT/GB2004/000433, a method is provided for producing a thin film transistor device incorporating a three-metal-level architecture resulting in a high aperture ratio and allowing for a large pixel capacitor. The device is formed by methods of solution processing and direct printing.
We here describe a method for patterning the upper pixel electrode on top of a multi-level pixel structure without significant degradation of the underlying layers of the TFTs.
In the prior art, it is known that conductive inks are able to be patterned using a wide variety of printing techniques such as ink-jet printing; screen printing, and offset printing to produce pixel electrodes or pads. However, these are relatively low resolution techniques that rely heavily on the wetting properties of the printing surface.
Conductive pads can also be directly deposited on top of a multilayer stack using a laser induced forward transfer process. This process can be executed with an infrared laser in which case it is known as a thermal transfer process. Conducting polymer pads have been thermal transferred onto flexible substrates as explained by G. B. Blanchet et al in Applied Physics Letters 82(3) 2003 page 463. However, this process uses a transfer layer that would need to disposed of after it is used, and it is unlikely that laser transferred conductive material would fill the via hole which would be required for it to act as a pixel electrode.
It is known to use ultraviolet laser radiation for patterning; this is described in US20030092267A1. This teaches the use of ultraviolet laser radiation, which is absorbed by most polymer layers and could cause detrimental effects to the performance of the TFT's.
The prior art discussed above has many drawbacks. When laser ablation is applied to patterning of conducting layers on a substrate that already contains active electronic devices such as TFTs, diodes, or other semiconducting devices or active layers, the performance of these devices/layers is degraded when the laser ablation occurs close to or in upper layers of the electronic devices. This is particularly problematic with many organic semiconductor devices. Many molecular or conjugated polymer semiconductors exhibit degradation effects, such as a reduction of charge carrier mobility or generation of electronic defect states, when exposed to strong ultraviolet radiation. Using ultraviolet radiation as described in the prior art for top level patterning for multilayer polymer stacks will tend to cause damage of underlying layers. In addition, such ultraviolet radiation patterning has been shown to produce debris. The formation of debris can cause poor contact with the display medium and may lead to shorts between conductive pads.
A method is therefore needed to define and isolate organic electrodes on top of a multi-level polymer stack within a device without harming the underlying layers or producing excess debris.